High density interconnect assemblages such as those described in U.S. Pat. No. 4,783,695, issued Nov. 8, 1988 in the name of Eichelberger et al., and in numerous other patents, are finding increased usage. In the typical HDI assemblage, a dielectric substrate such as alumina has a planar surface and one or more wells or depressions. Each well or depression extends below the planar surface by the dimension of a component which is to become part of the HDI assemblage. The component is typically an integrated circuit, having its electrical connections or contacts on an upper surface. These contacts or connections are preferably made from titanium-coated copper-containing metals, so that the later formation of through vias by means of lasers exposes titanium, rather than copper, to avoid oxidation of the copper, which oxidation might affect the adhesion of additional layers. Each component is mounted in a well dimensioned to accommodate the component with its contacts in substantially the same plane as the planar surface of the substrate. The components are typically held in place in their wells or depressions by an epoxy adhesive. A layer of dielectric material such as KAPTON polyimide film, manufactured by DuPont of Wilmington, Del., is laminated to the devices using ULTEM polyetherimide thermoplastic adhesive, manufactured by General Electric Plastic, Pittsfield, Mass., which is then heat-cured at about 260° to 300° C. in order to set the adhesive. The polyetherimide adhesive is advantageous in that it bonds effectively to a number of metallurgies, and can be applied in a layer as thin as 12 micrometers (μm) without formation of voids, and is a thermoplastic material, so that later removal of the polyimide film from the components is possible for purposes of repair by heating the structure to the plastic transition temperature of the polyetherimide while putting tension on the polyimide film.
Following the curing of the ULTEM adhesive layer holding the first sheet of dielectric film onto the components, through via apertures are made through the dielectric film and its adhesive layer at the locations of at least some of the electrical connections. The apertures are typically made by the use of a laser. The laser tends to generate soot as the dielectric and adhesive are vaporized. When the connections are made to copper surfaces, the heat of the laser action also tends to create copper oxides on the connections. The soot and oxides tend to prevent good metal-to-metal contact during later stages of processing which include metal deposition.
Following the drilling of through vias through the first layer of the polyimide film and its polyetherimide adhesive, a patterned layer of titanium/copper/titanium electrical conductors is applied to the exposed surface of the polyimide film, into the through vias, and onto the contacts of the components. This completes the formation of a first layer of electrical connections to the components. One or more additional thin sheets of polyimide dielectric material are layered onto the upper surfaces using silicone polyimide epoxy adhesive (SPIE) as a lamination adhesive. The SPIE is a thermoset material such as OXYSIM 600, manufactured by Occidental Chemical Corporation, Grand Island, N.Y. After application, the SPIE is then cured at temperatures below 200°. Once set, the SPIE cannot be softened by heating. Each additional layer of polyimide film has its own pattern of through vias drilled as far as the upper titanium surface of a lower layer of titanium/copper/titanium conductor, followed by its own layer of titanium/copper/titanium deposition. The titanium/copper/titanium layered metallized or deposited conductors are known to provide reliable interconnections.
It has lately become important to integrate into HDI modules certain components including copper-containing electrical connection materials. Such copper-containing electrical contacts are found in at least on-module connection strips, shielding or grounding members, and magnetic components such as tuned ferrite-loaded coils or transformers. These magnetic components tend to be somewhat larger than solid-state chips, but are dimensioned to be accommodated in the HDI modules for which they are intended. The integration of such modules presents some problems, in that the manufacturers of the components are accustomed to using copper as the main conductive material, and to making the electrical contacts from copper. Copper is not the best material for electrical contacts in an HDI context, because it oxidizes readily, especially in the presence of high temperatures. Neither titanium nor adhesives reliably adhere to oxidized or dirty copper. Even if they initially appear to adhere, the adhesion often fails in the presence of heat or moisture. Thus, a copper surface is not acceptable for HDI connection.
Other possible surfaces were evaluated for deposition on the copper of the magnetic components. Electrically deposited and electroless nickel, tin, and palladium were among the surfaces evaluated. It was found that adhesion of the lamination adhesive to nickel was relatively poor for both titanium and adhesive, regardless of how it was deposited. Tin was discounted as a suitable surface, because of the known problem of formation of dendrites. Palladium was also found not to provide good adhesion.
Thus, the presence of magnetics in HDI contexts requires improved adhesion. Feed-through electrical contacts containing copper require adhesion of the laminating adhesive to the surface of the metal contact. It has also become important to use titanium substrates in place of ceramics in HDI applications, and reliable adhesion to titanium substrates becomes more difficult as the titanium surface ages. Other metallurgies used in HDI contexts, such as aluminum bond pads, also require good adhesion to the lamination adhesive.
Improved HDI processing methods are desired.